text:
What is a gust alleviation system?
Reading about the 787 it says it has

an active gust alleviation system similar to the system used on the
  B-2 bomber. [1]

The gust alleviation patent says:

The gust alleviation system picks off the stick displacement signal
  and the aircraft normal acceleration sensor signal and processes this
  information. The outputs of the gust alleviation system are added back
  respectively to the existing flap command signal and the aircraft's
  original stability augmentation loop. [2]

As a layman I am not sure I fully understand this, so my question is, what is the gust alleviation system?

A gust alleviation system is a control system fitted to some Fly-by-wire (FBW) aircraft that reduces the effect of gust loads on the aircraft by deflecting control surfaces such as ailerons, rudder and elevators.
The system works by measuring the upward acceleration of the aircraft and comparing that with the acceleration commanded by the (auto)pilot. A feedback loop adds a correction signal to the signals controlling the deflection of the control surfaces in order to counteract the accelerations cause by wind gusts.
In its simplest form the accelerations are sensed near the center of gravity of the aircraft. More advanced implementations work with multiple sensors in the body and in the wings. These gust alleviation systems do not only attempt to annul the effect of the gusts on the aircraft's normal (upward) acceleration, they also reduce the wing bending moments. This in turn reduces metal fatigue.
In the A380 the load alleviation system is nicknamed "valse des aileron" (Waltz of the Ailerons) because of the dancing movements of the aileron:

So their movement is controlled? I've always thought that they are just swept by the wind :D

A320-100 used spoilers for gust alleviation, but the newer A320-200 variant and the rest of the A320-family don't.

Gusts change the dynamic pressure of the airflow over the wing. Vertical gusts even change the angle of attack. Both will change the lift the wing creates, and especially vertical gusts add stresses for the structure and discomfort for the passengers.
By changing aileron deflection, the wing's lift can also be manipulated. If the aileron deflection is just enough to compensate the added aerodynamic load due to a gust, both the wing and the passengers will benefit. By coupling the readings of accelerometers near the wing tip, the FCS can adjust the aileron deflection and ideally keep the wing bending constant.
Since the ailerons are at the outer 30% of span, they have a big influence on the root bending moment, but affect only a small part of the lift the wing can create. Therefore, while the bending moment can be nicely held constant, the lift will still vary, but less than without gust alleviation.
The part referring to the stick in the quoted patent refers to a mechanical control system with a naturally stable aircraft. If an aircraft with such a control system flies into a vertical gust, it will pitch up or down, which in turn will shift the equilibrium stick position. However, this will happen with some delay (the fuselage tip will "see" the gust first, and the tail will be affected by it some tenths of a second later), so the stick movement is a poor source for a feedback loop. I wonder why the patent mentions it - maybe the patentee wanted to cover all input sources.

@JanHudec: Could be. This leaves the question why they want to alleviate gusts with elevator commands when the aileron is at the source of lift creation.

I don't think it uses stick position for calculating the alleviation, but it might choose different solution depending on what command it is augmenting.

Do they want to use only elevator? I'd expect they augment primarily ailerons, but they also augment elevator to compensate any pitching moments the gust created or something.

@JanHudec: They say "flaps" in the patent. Given how slow the wing flaps move, this is really the worst way to do it. The patent is from 1990, and plenty more had been filed earlier. I guess Grumman needed to circumvent all other patents when they wrote this.

@PeterKämpf so the aircraft is still lifted/dropped bodily to a large extent?

@Abdullah: The wingtips are, but not its center.


text:
In a 777, what would happen if the auto-pilot and the altitude hold failed at the same time?
I know nothing about a Boeing 777 flight controls.
In general,
I know that the speed of an aircraft is controlled by the throttle on the center console. I have never heard of an automatic speed control. I believe the pilot manually adjusts the speed.
I have heard of altitude hold control. You set it and the system keeps the aircraft at the same altitude. Is this independent of the auto-pilot?
And of course there's the auto-pilot. It makes the aircraft go in a direction you want it to go.
If the pilot does not intervene, what would happen if the auto-pilot and the altitude hold failed at the same time?

Does the aircraft climb or dive?  Dose it oscillate up and down?
Does the aircraft roll over?
Does the aircraft turn?
What about speed?
If it continues in some fashion, what is the effect of the fuel consumption in the different tanks? Is it automatically controlled? What happens if that control fails?

Altitude hold is part of the A/P, and even when A/P is on, you don't fly without a pilot in the cockpit paying attention. If the pilot were not there and A/P failed, what would happen could be any of the above depending on the nature of the failure.

See page 8 of the Helios 522 report for an example of how an aircraft can behave once there is no active control input from autopilot or pilot.

Helios 522 was a pretty crazy example of absolutely everything going wrong.

The title asked about altitude hold, but the body talks about speed and auto-throttle.  It seems that the title should probably be changed to "...auto-pilot and auto-throttle."

Regardless of whether it's a 777 or any other airplane where the altitude is being held by the autopilot and the altitude hold function stops, whether because of general autopilot failure, just altitude hold failure, or the altitude hold function is inadvertently or purposely switched off; the airplane will seek whatever pitch attitude it is trimmed for.
For example, using the 747-100 and -200 autopilot setup, let's say you are in cruise at 35,000 ft, the autopilot is navigating and altitude hold is on. Further, let's say we're beyond 1990, and many operators of those aircraft had disabled autothrottles because they were a maintenance hog. Thus the airplane is maintaining whatever speed has resulted from where you have positioned the power levers. The altitude hold function is maintaining altitude by using the elevator. You can look down at an indicator on the center console between the pilots to see the degree of up or down elevator being used.
As the flight progresses and fuel is burned, the c.g. moves forward and the altitude hold function adds more up elevator to maintain altitude. When a certain amount of up elevator amount is reached, the system automatically retrims so that no up elevator force is required.  You hear and see it happen because the large trim wheel on the right side of the center console rotates.
If you're a good captain or a good flight engineer, you can monitor the amount of up elevator force being held and trim it out manually before it reaches enough to trigger the automatic retrim, and save the company a little fuel money (or if you're JFK to Tel Aviv in a -100, possibly avoid a fuel stop in Athens).
Now let's say that you reach up and disengage the altitude hold sometime between retrimmings. The airplane will nose down, and actually quite abruptly if it's been awhile since it last retrimmed. So abruptly that normal practice was to always have the yoke in hand before disengaging the autopilot.
Let's say that you that you let the condition persist. The nose is down, so the airplane would descend, of course. The speed would oscillate during the descent. If there was a severe nose down trim ... well, you wouldn't want to go there.
For a humorous account (at least meant to be humorous) of the altitude hold function being inadvertently disengaged, go to http://terryliittschwager.com/hajj4contest.php.
Now for the case of total autopilot failure, I defer to the other answer already here when I posted this. Suffice it to say that the pilots would have to take control in short order to avoid an extremely serious condition, very possibly the loss of the aircraft. You can go to http://community.seattletimes.nwsource.com/archive/?date=19911219&slug=1323954 for information on an incident that involves such. The interesting thing is this airplane had started to do this a number of times (including once to me coming out of Hong Kong), but in previous instances the pilots quickly recognized that the autopilot had failed and had taken control before things got serious.
Following the incident described, the airplane was repaired and reentered service, and we called her Christine.

I don't think I'd want to fly on Christine :/

No, speed is not controlled by "throttle" on the center console1. Due to the way aircraft longitudinal stability works, the elevators (controlled by control columns (yoke)) control speed and thrust or power2 levers control climb rate.
Automatic speed control, "autothrottle" (A/T) is a cockpit automation mode that adjusts thrust to maintain speed. Since thrust does not control speed, it relies on someone operating the elevators to maintain flight path or at least attitude. This would normally be autopilot3.
Altitude hold is one of the modes of "autopilot" (A/P). Autopilot controls both pitch and roll. Via pitch it can control vertical speed or altitude and via roll it can control direction. The term autopilot may be used including or excluding A/T.
At altitude it becomes increasingly difficult to fly by hand. To allow lower separation, aircraft are only allowed above FL290 if they are equipped with working altitude hold and pilots would normally use it.
That does not mean the pilots are not watching the instruments. In case the autopilot fails, they would resume flying manually.
If the pilots did not assume control, than the aircraft would start a Phugoid oscillation. It would continue straight, but since the lateral stability is rather weak4 if turbulence caused it to bank to the side, it could overbank and enter Graveyard spiral.
Normally engines only take fuel from their side5. But both engines are normally consuming the fuel at the same rate and the initial amount is the same on both sides, so they would both run out quickly after each other.

1Except if you engage the autopilot vertical speed or altitude hod mode or in Airbus with it's normal law which automatically retrims the aircraft for current speed to maintain flight path which is similar automation but built into the system and never disengaged except in case of failure.
2"Throttle" refers to the flap restricting airflow to the engine, which is specific to spark-ignition reciprocating engines. Compression-ignition (diesel) engines and turbines are controlled by directly adjusting the fuel flow, don't have throttle and thus the levers are not usually called "throttle" in them. They are called "power" levers for propeller engines (turboprop and diesel) and "thrust" levers for jet engines, reflecting the difference in the engine behaviour with increasing speed.
3While A/P and A/T can always be engaged independently, in most aircraft it is recommended to always used both or neither. Exception is again Airbus where A/T without A/P makes sense.
4Making the aircraft laterally stable is possible, but unfortunately it aggravates the Dutch roll mode. So usually the lateral stability is limited and will only return the aircraft to level from small disturbances.
5I am not sure whether on B777 the centre tank feeds the engines directly or transfers to the wing tanks, but the result is not much different.

If you disengage the auto-throttles, but leave the rest of the autopilot operating including the altitude hold function, and then you increase advance the power levers (or whatever you choose to call them), the airplane's speed will increase. If you then retard the power levers, the airplane will slow. Thus, have you not controlled the speed with the power levers? The argument over whether it is best to think of the elevators controlling speed and the power controlling altitude or vice-versa is an old one. In reality you can control either with either.

@Terry What you say is true, but only because the auto-pilot is giving input to the elevators and/or elevator trim in order to maintain altitude. If the input to the elevators and elevator trim is held constant, advancing the thrust levers will cause the plane to ascend, not speed up. Likewise, pulling them back will cause the airplane to descend, not slow down. Of course, you use both controls together to achieve the desired altitude and airspeed when flying correctly, but Jan is correct regarding the effect of changing the thrust setting without giving elevator or trim input.

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text:
Why is the flight going north just to go south?
I am tracking my kid's flight from LAX to EWR. It seems like a pretty straight West to East kind of flight but the flight path has the plane moving somewhat north through Ohio and then angling back to the south
Why wouldn't the flight just keep going East?

The Earth is a sphere (to a first approximation), which means that the shortest path is not what it looks like on a map. This is the actual shortest path, and you'll see it fairly approximates what the flight is actually doing (the variation is due to the fact that planes fly on straight paths between predefined waypoints which aren't designed for smooth turns for every possible routing).

@waiwai933 That should be an answer, not a comment. Comments are for asking for clarification and so on, not for answering the question.

The path is a Geodesic, see this - http://www.astro.umd.edu/~richard/ASTRO340/class11_RM_2014.pdf

@AwalGarg - the map shows a sharp corner in the path.  Geodesics don't have sharp corners; there's something else going on here.

@DavidWallace It is a Geodesic. It is looking a sharp corner in that plane. It is not so in real...

@AwalGarg the path isn't Geodesic.  Airplanes (especially in the east) are not cleared direct to their destinations, which is the only way they would be on a true great circle.  This path is just a result of the points along his cleared flight plan.  See my answer below for details.

@casey Ok, it isn't a Geodesic. But, can you also clarify if it really matters if it has a sharp corner? And does it have one in real?

@AwalGarg Airways are defined by waypoints and especially when transitioning airways, you can end up with large turns over certain waypoints.  In the routing above, that sharp corner is real and is the transition between two airways.  Unless the airplane gets a further direct clearance (e.g. direct SLT), it will fly that dashed route, including the corner.

@casey Do you think an airplane can make a sharp cut?

@AwalGarg most enroute waypoints do not require you to actually fly over them and the flight director will start the turn early enough that it can be completed at the proper turn rate and roll out on the new course.

Aside from the storms causing an earlier re-routing, that jog north is to join the arrival into EWR.  A common fix in NE Ohio on flight plans into EWR is the DJB VOR which is in Loarine county (I think, somewhat near Cleveland). and then routing over Slate Run and then Williamsport, PA on the way into Newark.  If you are a jet coming from the west, this is literally the only way into EWR.  The other two ways in are from the south joining somewhere on a line between Atlanta and Baltimore (which you can transition to from the west by going over Beckley, WV) or from the north coming in over Albany, NY.  The primary reason for the strict routing starting at around the OH/PA border is the need for multiple arrival descents for both jets and turboprops into the various NY area airports (EWR, JFK, LGA, TEB, etc), the westbound departures out of NY center and the letters of agreement between Cleveland and NY centers to manage these traffic flows.
See this image:

This represents a route of ..J29.DORET.J584.SLT.FQM3.  This is not the exact route a LAX originating flight would normally get as they would end up a bit further north (going over Detroit instead of Cleveland), but they would still converge on J584.SLT.FQM3 (the portion east of the OH/PA border).  No matter your position west of the OH/PA border, Cleveland center is going find a way to put you on J584 before you get very far into PA.  This is why the flight couldn't just go direct to EWR and had to first fly north.

Part of the answer is that, as noted in waiwai933's comment, the earth is (roughly) a sphere, not a flat plane. This means that the shortest distance between two points is not a straight line on the map but what's called a great circle. To find this, you'd take a circle whose centre is the centre of the earth and which passes through the two points you're trying to connect. The shorter route along that circle is the great circle distance.
The intuitive reason for this is that the earth is "narrower" closer to the poles so you can gain a lot by flying towards the pole, using the "narrowness" and then flying away from the pole again. The "ideal" route from LAX to EWR is this (images from gcmap.com):

A much more pronounced version of this would be if your son was flying to Casablanca instead of Newark.

Although Los Angeles and Casablanca are at roughly the same latitude, the shortest route between them actually goes over Canada, over a thousand miles to the north of LA!

As far as not being a straight line on a map, that really depends on which map projection you're using. The gnomonic projection shows great circles as straight lines. On the other hand, your 'typical' (Mercator?) map isn't going to do that.

If you have a globe, an easy way to find the great circle path between two given points is to put a piece of string on the globe, one end at each endpoint, and stretch the string such that it presses against the globe; it will then trace the great circle path. Of course on a real-life flight, there will be other considerations as well, but to a first order approximation (same as the Earth being spherical) this method works well.

'the earth is "narrower" closer to the poles' -- to be precise, the Mercator and other cylinder-based projections are more stretched closer to the poles. The extreme being that the single point of the north pole itself is stretched out to form the entire top edge of the map. You can "teleport" instantly anywhere you like along that line, if you're willing to fly over the pole. That's also why Greenland (near the pole) looks roughly the same size as Africa (straddling the equator) on a Mercator map, despite in fact being about 1/15 the area.

@SteveJessop What you say about map projections is absolutely correct and very relevant but the earth really is narrower closer to the poles. You can view it as a stack of slices parallel to the equator and the radius of those slices gets smaller as you approach the poles, since the earth isn't a cylinder. In fact, the radius decreases slightly faster than it would even if the earth were a sphere, which makes it even more advantageous to go closer to the poles.

@SteveJessop: Mercator maps elongate north-south distances toward the poles just as severely as east-west distances.  The map distance between 70N and 83N would be roughly comparable to the distance between 70N and 70S or between 83N and 86N.  A Mercator map which stretched from 86S to 86N would need to be more than five times as tall as one which went from 70S to 70N.

@SteveJessop "Most Mercator maps ..." ??  The reason why ALL Mercator maps fail to reach the poles is that we don't have an infinite supply of paper.

@DavidRicherby: it's true that you can view the earth that way, but of course you could also view it as a stack of slices parallel to the (extension of the) Greenwich meridian, and the "narrow" region would be elsewhere. This effect isn't "really narrower", it's just about the projection. It's also true as you say that there's an approx 7km "narrowing" because the earth isn't spherical, but that's a very weak effect in comparison. all: thanks for the correction that Mercator can't depict the pole.

@SteveJessop According to Wikipedia, the polar radius is about 21km less than the equatorial radius.

Actually, that flight was first routed South of its original flight path, in order to avoid a very large storm system in the mid-west. At that point, it still could not head straight to Newark, due to another storm system in Indiana.
They were originally flying a little more East, but then got routed a 'smidge' North, to avoid the system, most likely when the system didn't move as fast as they figured. As to why they kept heading North-East, instead of a more direct path from Indiana, I cannot tell.
If you (quickly) look at the flight on FlightAware.com, you can see the course corrections due to weather.
scratch the following, the TFR (temporary flight restriction) isn't until later this week.
Another factor for this flight, was that it looks like once they went North to avoid the weather in Indiana, when they came east of the weather, if they'd have gone straight towards Newark at that time, there is a NOTAM for a TFR in Western Pennsylvania. So they most likely were shunted farther North by ATC in order to avoid the TFR.

@Articuno Thanks. The irony, of course, is I'm the one who brought up acronyms in meta... d'oh

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text:
How did the term 'Buttered the bread' originate?
I've heard this term used a few times to describe an exceptionally smooth landing

"Exceptionally smooth" landings are not necessarily good ones. I've heard these "gentle" touchdowns referred to as "like a butterfly with sore feet", but firm touchdowns are necessary depending on conditions.

Closely related

Back when actual butter was the main spread to put on bread, it took a very gentle touch to spread butter straight from the refrigerator (icebox) without tearing up the bread.  Thus, a very smooth landing might be said to have "buttered the bread" because it went on smoothly enough to keep from tearing anything up.
This probably also evolved into "greased it on" or "greaser" to refer to a similarly smooth landing.

Oh how I remember this... while I did not like toasted bread it was much easier to spread cold butter on warm toast.

What is the main spread to put on bread now? In my experience, your first sentence (except the icebox part of course) is still true.

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text:
Aviation history. My dad PM of F-14
My father was Prog mgr of F14. Where can I sell or donate his memorabilia or where is site to help?

You can sell almost anything on eBay. If what you have is seriously collectible then the big auction houses (Christie's, Sotheby's) might have an appropriate event where you could list it. This isn't really an aviation question, though.

I would suggest that you contact the Naval Aviation Museum at NAS Pensacola FL to discuss donating or loaning them.  That way his legacy can live on with a plaque stating "Items in this display donated by ___, NAVAIR Program Manager F-14 Tomcat Systems, 1972 - 1986". (or something similar)


text:
What will happen to wing Vortices on supersonic aircraft?
Having looked at the new Aerion AS2 design I wondered whether anyone could tell me what happens to wingtip vortices at a cruise of Mach 1.4 with the strange wing shape of the AS2 design? I know that induced drag (very closely associated with wingtip vortices) reduces with velocity, and that many supersonic aircraft work by vortex lift using delta wing configurations, but the AS2 has bizarrely conventional stubby wings. can anyone help?

Induced drag is not associated with wingtip vortices.  Induced drag is associated with the lift-generation process deflecting air downwards when it was previously going straight backwards, thus decreasing the backwards-going part of the air's motion and pushing backwards on the aircraft.


text:
What is this gigantic dish at Ben Gurion airport?
As the title asks, what is this thing for?  Have never seen anything like it at other airports I've been to.

Is it just for decorative purposes by any chance?

It's probably some insane technology hiding in plain sight. That would be soooo Israel...

Totally just a decoration. Fire at will, commander.

It would be a great place for an endurance bike race, "The Long Walk" style. If you're too slow, you fall from the velodrome, down into a pit.

Skate rink, of course!

It is just a design feature, and serves no technical purpose, beyond filtering some light and providing a waterfall feature.

Safdie designed the roof of the rotunda as an inverted dome, an asymmetric dish floating above the piazza that lets sunlight in and filters the daylight with a special white fabric ceiling.
source: https://www.bdcnetwork.com/ready-takeoff

and

Rain falling upon the suspended dome drains towards the center and enters the rotunda through an oculus skylight as a waterfall. In the dry season, a continuous flow of water washes the roof, contributing to the rotunda’s cooling and forming a fountain through the oculus.
source: https://www.safdiearchitects.com/projects/ben-gurion-intl-airport-airside-terminal

Image, because it looks nice (Same source as second quote above)

"that lets sunlight in and filters the daylight with a special white fabric ceiling" well that is a technical purpose, won't you agree?

@DeepSpace That seems to be the purpose of the 'special white fabric ceiling', not the dome design itself

@Rob: But still the dish serves a technical purpose just like any other possible roof for the building. The question could have been simply answered by: The dish serves as roof for the building ;-)

It's Architecture [sic], and it's for your eyes to behold:
Safdie architects explain further.

Why did you write [sic]?

Because this is architecture with a capital letter :)

This is the decorative roof above the duty-free area, which is circular with a plaza in its center. From the top center and around the "sun roof" there a circular waterfall.


text:
Are there any aids (like a padded "belt") that can ease hip discomfort during aerobatics?
When flying aerobatics, particularly maneuvers such as a slow roll, I frequently experience rather high discomfort on my jutting hip bones.  I've googled "padded belts" but I have not found anything that would seem to help.  Is such a thing an actual product, and if so, is there a proper term I could search for?
Basically, I'm looking for a garment/belt I could wear that would provide sufficient padding around my hip bones to mitigate this discomfort.  (The discomfort is caused by the tight seatbelt combined with negative G's -- the belt itself is standard issue on a Pitts Special.)

You can try out a padded harness which may help, a variety of places mention the stuff offered by Hooker Harnesses and may be what you are looking for.

(source)
If you currently only have a lap belt installed in the aircraft in question you may want to consider 5 point harnesses as they can help by moving some of the load to your shoulders.

Since you mentioned a Pitts, I assume you already have a 5 point or 7 point harness (a 7 point is just like the 5 point, but with the addition of a second lap belt).
Check to make sure you've got the lap belt as low as possible.  It should be below your belly, not on it.  The whole idea is that the pressure of the belt is shared between the tops of your thighs & your hips.
When you're strapped in, if you roll inverted you should be hanging comfortably mostly from your thighs.
It might be necessary to tighten the strap that goes between your legs.  That will help position the lap belt so it's low enough.
FWIW, that strap is the "anti-submarine" strap - it exists to keep the lap belt on bones & not on squishy lower abdomen.
You might try stuffing some foam under the lap belt, but my experience is that once I have the lap belt properly tightened, the foam is smashed flat and fairly useless.


text:
If an airline cancels an aircraft order, can a different airline acquire those options?
Let's say Airline A placed an order for an aircraft a while back. After a bit of time, Airline A decides it no longer has a need for the aircraft and decides to cancel the order. Meanwhile, Airline B decides it wants to buy said aircraft but doesn't want to go to the back of the delivery line. Is there a process where Airline B can take Airline A's spot in the delivery queue? I vaguely remember seeing something like this happen before, but cannot find any sources.

Anything is possible but remember that a purchase contract is between the mfr and the buyer. Airline A cannot transfer the contract obligations to airline B (because the mfr has an obligation to A and cannot usually transfer that obligation on the mfrs behalf), airline B would have to negotiate the queue position with the mfr when it purchases or assumes the contract. Mfrs may use this as a selling tactic when somebody cancels a contract.

In general yes, but depending on how close the aircraft was to completion they have to live with a different cabin and cockpit layout than what they are used to or would had ordered themselves.


text:
How do you calculate trip fuel based on hourly fuel burn rates?
I am working on a small personal project that uses fuel burn data about specific aircraft to calculate the total fuel required for a trip. The data looks like this:
          | 120000 kg  | 150000 kg | 180000kg
----------+------------+-----------+-----------
21,000 ft | 6250 kg/hr | 6350 kg/hr| 6450 kg/hr
22,000 ft | 6225 kg/hr | 6325 kg/hr| 6425 kg/hr
23,000 ft | 6200 kg/hr | 6300 kg/hr| 6400 kg/hr

Based on the table above, you can see that to cruise at 21,000 ft and a weight of 120,000 kg, you would burn 6250kg/hr.
The problem I am having is that in order to calculate total weight, you first need to know total fuel, which you don't have until you know the proper burn rate. What would be the proper way to calculate fuel based on this information?

Generally, you don't!  There are different charts and/or tables (and computer programs) which are used to calculate the amount of fuel needed for the flight.  They generally start with your landing fuel (at the destination / alternate if required) and work backward to come up with the required ramp/takeoff fuel and cruising altitude.

Do you also have the data for fuel to altitude? You can't calculate trip fuel without those.

This would typically be solved iteratively. You start with your mass equal to ZFW and calculate how much fuel you need based on the table:
$$ \mathrm{Fuel}_1 = \mathrm{Fuel}(m=\mathrm{ZFW}) $$
Then you add this fuel to your total weight and repeat the calculation:
$$ \mathrm{Fuel}_2 = \mathrm{Fuel}(m=(\mathrm{ZFW} + \mathrm{Fuel}_1)) $$
which will give you slightly larger result. You repeat this process until convergence, meaning
$$ \mathrm{Fuel}_{i + 1} \approx \mathrm{Fuel}_i $$
and this is your final result.

I think you're using the data the wrong way round. Fuel burn does not dictate how much fuel you should carry, but it might dictate what altitude you want to cruise at to conserve the maximum amount of fuel.
The amount of fuel you need to carry is based on endurance plus contingency. If you know the endurance of your aircraft is 3hrs, and your planned route estimates at 1.5hrs having added the appropriate contingency you might decide that you need 2 hrs of fuel for the journey. That means that you need tanks 2/3 full.
Now you know that the fueld to 2/3 fill your tanks weighs x, the weight of your aircraft is y  then the total weight is simply x+y.
Using your datatable, you can now calculate the optimal cruising altitude.
Obviously this is oversimplified, as you still have the problem that it might be worth carrying slightly more fuel to get a better cruise efficiency.

Wouldn't endurance change based on fuel burn though? My endurance would be much longer if I was burning 5,000 kg/hr versus 8,000 kg/hr.

Problems like this are more easily worked backward than forward.
You know how much fuel you should have at landing (IFR reserves) and how much the plane will weigh with that much fuel.  From there, calculate how much fuel you needed to have at 1 hour before landing to get that result, then an hour before that, etc. until you get all the way back to the takeoff, plus whatever fuel you expect to burn for startup and taxi.

The normal approach is to segment your flight so that you know your burn for different phases of the flight.  For example, cruise after a long flight, may be at a reduced power setting, because the aircraft is lighter (having burned much of it's fuel).
Then one adds up the fuel for each segment, including reserve, to get the total load.
Manually, iterative refinement can be done to optimize things.
For the flying I do, there is no push to drop the fuel load down, so we can pick a point, and with one run at he fuel (manually when not using an aircraft with FMS or electronic aids (tablet), we can verify if the initial guess is close, and decide to use that load.
However, friends who fly for airlines say that the ops people are always cutting their fuel tight, especially when there is weather which could cause diversions.  They will ask for a few more pounds "for the wife and kids."  Then again, a fuel diversion can be expensive.
Another point, normally one selects cruise altitude for WX conditions, traffic, and flight duration.  In other words, normally the available fuel does not drive altitude.  Altitude is driven by flight conditions, routing, economy, etc.
So in summary, segment your flight, and use climb and descent burn rates and rates for your weight, power (or TAS) and altitude, to get an initial "proposal" and then iteratively refine if necessary.  Obviously add reserves and a flight condition based reserve.  If you will be picking your way through some weather you will want a reserve for that.

I ended up solving it by programmatically going through each and every weight/altitude combination, multiplying the fuel burn rate by my duration, and adding that weight to my ZFW. If I was below MTOW, I saved the fuel burn rate and altitude, otherwise I discarded it. At the end, I picked the lowest fuel burn rate out of my saved options, which also gave me my optimal cruising altitude.

From a calculation standpoint, brute force with all values, takes more work than an iterative refinement.


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How to design a propeller that works efficiently in reverse?
I'm designing a UAV that needs to reverse motors for some maneuvers, but unfortunately most of the commercially available propellers I've tested are optimized for single direction and only produce approximately half the thrust when spun in reverse.
If I wanted to design my own propeller with better "reverse thrust" than 50%, what considerations could I make?

What you can do with a UAV is pretty different than what makes sense with full-size aircraft, but for turboprops that can go into reverse, they do it not by reversing the direction of rotation of the engine, but by changing the pitch (blade angle) of the propeller. It keeps spinning in the same direction; halting & reversing the momentum of an engine + a large propeller wouldn't be a quick process at all, and the mechanism to change the blade angle is compact enough. Plus you need to change the angle anyway, so this just lets you change it a bit farther.

Since the considerations here are specific to UAV's and pretty distinct from non-uav Aviation, this may be a question to migrate to that forum rather than keeping here.

Amateur here but I feel at the end it's going to be a tradeoff between "trying to minimize loss of power in reverse mode" and "having more avalable forward thrust". Unless you can find a way to QUICKLY rotate the prop blades by 180 degrees, then the same thrust vector is just facing the other way, still at its best. A little bit like a constant speed prop changing it's blade angle but even in that case the reverse thrust is always lower than max forward thrust.

There are electric indoor radio controlled airplanes capable of doing "3D Aerobatics".  This is what you want to search for.
They use a variable pitch propeller set up like a tail rotor on a helicopter.
I.e. it uses a hollow shaft motor and puts the swashplate on the 'other' side of the motor.
The other keyword to search for is "EVP" for Electric Variable Pitch.
Something like this.

3D rc flying generally does not involve variable pitch - mostly just post-stall maneuvering. Perhaps 4D is what you were meaning to say?

Quite possibly.


text:
How do fighter jets maintain control while wings are perpendicular to airflow direction?
In my understanding, a plane stays stable and "straight", largely because the control surfaces such as the tail fins keep it stable by applying force in the opposite direction of any unwanted drift.
However, I have seen fighter jets and even small RC aircraft, perform a maneuver to reduce speed, in which they rapidly pull up until they are vertical and flying perpendicular to airflow.  While this looks cool, my question is: how does the aircraft stay upright and stable during this maneuver?
What is preventing the aircraft from flipping to its side or beginning a spin?

I'll be mostly talking about high AOA lateral-directional static stability here. An important aerodynamic parameter to look for is the Cnβ dynamic, which provides a rough indication of the oscillation frequency of uncommanded roll-yaw motion, and whether a nose-slice type of yaw departure is possible at the current AOA.
The figure below shows such parameters typical of a spin-resistant twin-tailed swept-wing fighter, and how Cnβ dynamic is calculated.

Another example of the F-16A from NASA wind tunnel data:

Directional static stability derivative (Cnβ), or yawing moment coefficient (Cn) with regards to sideslip (β), shows which direction the aircraft would yaw if sideslip is present. A positive value means the aircraft tends to return to zero sideslip if unbalanced. Cnβ usually becomes negative at medium AOA ranges of 20-30 degrees, as the vertical tails are increasingly immersed in the wing/forebody wake.
Then there's the dihedral effect, or lateral static stability derivative (Clβ). It is the rolling moment coefficient (Cl) with regards to sideslip (β). A negative value means a positive dihedral effect, that the aircraft tends to roll into the yaw if sideslip is generated. This dihedral effect is also usually at its weakest at medium AOA ranges.
There's also asymmetric breakdown of the vortex flow at a medium AOA range of 30-50 deg, that can contribute to roll and yaw moment, so what you want to do is to pitch through those unstable AOA region as fast as possible, that the asymmetry in the flow pattern has little time to be formed.
Thus, an effective way for reducing lateral-directional disturbance at the post-stall regimes is to increase pitch rate and reduce the time staying in these AOAs full of separation phenomena and asymmetric flow patterns.
By reaching the high AOA range of 60-90 deg, the flows are fully separated and Cnβ dynamic tends to increase.
This is actually how a Cobra maneuver is possible with little roll-yaw motion by the Su-27. Note that I did not mention longitudinal stability here, but you can refer to this answer to see how such maneuver is aerodynamically possible in the pitch axis.

Good discussion. Differential horizontal tail deflections can create (or counter) some body axis yaw at higher angles of attack. Moments of inertia about the pitch axis (Iyy) and yaw axis (Izz) tend to be fairly high for aircraft demonstrating these maneuvers. I think another key point is as these air vehicles are transitioning to higher AoA's, it's important to keep roll and yaw rates small upon entry.

WRT "stay upright", it's a balance of moments about the pitch axis, including those created by aerodynamic forces, propulsion system forces, and inertia. Many configurations have horizontal tail designs which retain some effectiveness at high AoA. Thrust vectoring can make a big difference in pitch, especially at slower speeds/higher power settings.

@AeroAndy Very true. Differential horizontal tails are indeed very effective yaw devices at higher AOAs, making use of the adverse yaw created by asymmetric induced drag. It's so effective that non-TVC aircrafts such as F-18, F-35, as well as TVC aircraft such as F-22 (with 2D TVC of no yaw capability) actively deploys such control logic that increase yaw controllability at medium to high AOA.

Thank you for the informative response -- it was definitely very interesting.  But you didn't mention in the end, what design features affect the Cnβ?  What makes an aircraft perform better in this parameter?

@CaptainCodeman This would subject to the design of leading-edge flaps / slats, vortex generators, wing-fuselage and vertical tails. A wind tunnel is required to make such comparison by uninstalling a particular component and spot the differences in Cnβ and Clβ, but generally speaking, deploying leading-edge flaps / slats would positively affect Cnβ and Clβ in most AOA ranges. A higher aspect ratio and a larger area of vertical tail would positively affect Cnβ at lower AOA, but cannot guarantee an increase in Cnβ at high AOA. I would recommend a read of the paper NASA TN D-5361 for an example.

If you are talking about this:

how does the aircraft stay upright and stable during this maneuver?

It does not.  The aircraft is rotating.  You will not see an aircraft maintain this angle of attack for several minutes.  Typically it is closer to milliseconds.
Computer controlled aircraft do not need to be stable.

(The angle of attack in this video is less than 90 degrees.)


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How do planes deal with excess lift at high speeds?
If I understand correctly, wind force increases quadratically as airspeed increases.  Therefore, all other things being equal, lift should also increase quadratically.  Does that mean, a plane going at 600 mph has 9x more lift than when it was going at 200mph?  How can the flight be stable at both speeds?  Surely it will either have too much lift or too little lift at one speed?

By trimming. That's what the trim wheel is for

The lift generated by wings is proportional to the square of the calibrated airspeed and proportional to the angle of attack up to a point.  As an airplane accelerates, it must reduce its angle of attack to remain in level flight and avoid climbing.  So an airplane traveling at 300 knots of calibrated airspeed will fly at a much lower angle of attack, but still produce the same lift force, as an airplane flying at 100 knots of calibrated airspeed.
If an airplane slows down, the pilot must increase the angle of attack in order to generate enough lift to fly. If they continue to slow down, at some point the angle of attack required to maintain enough lift for level flight will cause a disruption of airflow over the top of the wings. This is the critical angle of attack beyond which the wing will suffer an aerodynamic stall.  If the wing suffers an aerodynamic stall, the amount of lift drops off dramatically for an increase in angle of attack, and the pressure drag dramatically increases. At this point, the airplane can no longer continue to create enough lift in order to fly.

It's actually pressure drag which dramatically increases during stall. Lift can, but must not always, drop dramatically in post-stall condition.

Thanks for this.  Is there some tool that simulates/calculates the lift of an airfoil as a function of the angle of attack?

@CaptainCodeman  To a good first order approximation, lift is proportional to angle of attack at any given speed, linearly, lift = ka, where k is a constant, and a is the angle of attack, when a is between the positive lift and negative lift critical angles. Small deviations in that approximation are handled automatically by the pilot simply adjusting the angle of attack to give the required aircraft acceleration. There is so much variation in the speed squared term they have to handle, that a minor deviation from angle linearity is irrelevant.

@CaptainCodeman The relationship between lift and angle of attack is something that is commonly provided, although it's usually given in terms of lift coefficient. For example, take a look at airfoiltools.com - on every airfoil's entry you'll see a graph of Cl vs Alpha, where Alpha is a common synonym for the AoA. With the lift coefficient, you can calculate the lift of any given wing using that airfoil; Lift = wing area * air density * (airspeed squared / 2) * lift coefficient. That graph is usually determined by experiment or CFD simulation.

But doesn't a cambered airfoil provide lift even at a 0 angle of attack?  How does that work with the lift=ka formula?  Is the camber added to the angle?

@CaptainCodeman The definition of the exactly zero angle of attack in those airfoils is somewhat arbitrary. The formula uses a as the angle from the zero-lift angle of attack, whatever the position of the chord line happens to be. So you just shift your angles so that you have zero lift at zero angle.

If the plane slows down, angle of attack would need to increase to maintain level flight.  A plane that's in a sufficiently deep stall might be described as "no longer flying", but the only reason a plane would need to nose up in order to "keep flying" would be if it would otherwise collide with something (like the planet below it).

If I understand correctly, wind force increases quadratically as airspeed increases

Correct. Anyway we speak more correctly of "aerodynamic" force.

Therefore, all other things being equal, lift should also increase quadratically.

If everything remains exactly the same then yes.
Anyway this is not how an airplane deals with higher speed. The aerodynamic force depends not only on the speed but also on:

the fluid density
the dimension of the aerodynamic body
the shape of the body
the angle with which the airspeed hits the body, more or less in a linear manner. This angle is termed "angle of attack" and called $\alpha$. Comprimibility effects (Mach > 0.3) and structural deformations modify this simple linear relationship but the main idea still holds.

In general the equation expressing an aerodynamic force looks something like this:
$F=½ \rho V^2 S C_F$
Where:

$\rho$ is the fluid density - see previous point 1.
$V$ the speed
$S$ a surface which is used as reference area (for an airplane it is normally the wing area, for a car is the frontal surface) - see previous point 2.
$C_F$ is the part depending on the shape of the body and $\alpha$ - see previous point 3. and 4.

If for example we consider the lift, then that equation becomes:
$L=½ \rho V^2 S C_L$
Where $C_L$ (lift coefficient) has the following shape (underlined in blue):

As you can see, $C_L$ increases linearly with $\alpha$ (till a certain point called stall) and can be used to change lift as needed.
Let's do a simple example: if the speed doubles, its effect on $L$ becomes four time as big. If we reduce $\alpha$ from 10° to 2° then the $C_L$ decreases from 0.8 till 0.2 i.e. four times, compensating for the increase in $V$.

Some aircraft deal with excessive lift by having a "negative flap" setting, where the flaps are positioned up a little bit.  Some gliders do this to reduce drag caused in high speed flight by their high lift wings.
Reference: Do negative flaps increase glide ratio?

Another aspect to consider is control sensitivity. Excessive lift at higher dynamic pressures usually increases load factor sensitivity to pitch inputs. According to MIL-STD-1797A Flying Qualities of Piloted Aircraft, there're requirements that must be satisfied when comes to handling qualities at all dynamic pressures. One of the parameters to look for is the $n$/α, or the steady-state load factor change per unit change in angle of attack for an incremental pitch control deflection at a certain airspeed.

When dynamic pressure increases, $n$/α usually increase as well because load factor $n$ = Lift / Weight, and $n$/α = Lift / (Weight * $\alpha$) = (CL$_α$ * α * dynamic pressure * S$_{ref}$) / (Weight * α) = CL$_α$ * dynamic pressure * S$_{ref}$ / Weight.
CL$_α$ is the slope of CL curve vs AOA that will also vary slightly w.r.t mach number.
In order to keep a satisfactory $n$/α at both low and high dynamic pressure, the gear ratio of stick input to elevator command is usually reduced with increasing dynamic pressure in CAS (control augmentation system) equipped aircraft, that reduces control sensitivity. An increase in stick force per g will also positively affect control sensitivity at higher dynamic pressures.

authors: Vladimir F Героям слава


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What are the main subjects covered in an aerospace engineering degree course?
Suppose someone wanted to self-study by reading some of the main textbooks that are covered in an undergraduate degree on aerospace engineering.
What are the main topics that are covered?
(As reference, I could say that the main topics I studied for computer software engineering were: Calculus, Physics, Engineering Mathematics, Electrical circuits, Digital Circuits, Discrete Mathematics, Graph Theory, Automata and Languages, Algorithm design and analysis, Java/Python/C++, Systems Design, Computer Languages, Computer Organization, Operating systems, Compiler design, Artificial Intelligence, Computer Graphics.  I want a similar list for aerospace engineering.)

For example: https://student.mit.edu/catalog/m16a.html

@sophit, That's useful, thank you

Another curriculum example from CU Boulder:  BS AES Curriculum with a link to the complete course catalog.

Every university will have its course catalog available online.  This will include the degree requirements for every major including short (one-paragraph) descriptions of every course offered.  You may also be able to find individual course websites (if the instructor makes one public) that includes a course outline, with a listed textbook, topics, and sometimes even homeworks and projects.
In the US, engineering schools are accredited through ABET (www.abet.org), which has criteria for engineering in general as well as subject specific requirements.  Their criteria are listed here.
There are three distinctions that are important -- aeronautical, astronautical, and aerospace.
The vast majority of programs in the US are 'Aerospace'.  This covers both aircraft and spacecraft applications.  Aeronautical restricts itself to aircraft applications and astronautical restricts itself to spacecraft applications.
The Aerospace standard is basically that it must contain everything from either aero or astro -- and 'some' from the other.  Consequently, most 'aerospace' programs are strongly tilted towards either aeronautical or astronautical.  (Most of them tilt aero).
If you are looking for a school to attend and you know for sure that you want to focus on air/space, then you should do some deeper research into the schools you are considering and figure out if their focus aligns with your interests.  If you are unsure, then try to find a school that provides a rich experience for either track.  This may seem counterintuitive, but you want as many courses split as possible.  Note, even though you can delay your decision a little, you will need to decide in the first year or two of university.  In the first two years, you are mostly taking foundation courses (Math, Science, etc. followed by basic Engineering - Statics, Thermo, Dynamics, etc.) and you don't get into your Major specific disciplinary courses until 3rd and 4th year.

Thanks for your response.  I did an undergraduate in Computer (Software) Engineering 20 years ago -- I'm trying to see if I can self-study aerospace engineering sufficiently to hop straight onto a graduate degree -- or possibly reduce the duration of a full new undergraduate degree somehow.  If you have any advice I would be grateful.

I would encourage you to look at software jobs at Aerospace companies.  Aircraft and spacecraft are basically flying computers these days and there are lots of interesting software challenges across the product lifecycle.  Anecdotally, aircraft companies struggle to recruit software people (because they all think the good jobs are in Silicon Valley) -- while their jobs for 'aerospace engineers' are very competitive.  Once you're at an Aerospace company and you see the various roles, reconsider more education -- they'll probably pay for it.

Thanks Rob but I've already made enough money to have no need for a job; I just wanted to learn more things in a setting surrounded by other people as self-study is limited.

In that case, it may be complicated.  These days, many universities don't have a straightforward mechanism for a person with a BS to apply to a BS program.  If you do, they don't know how to give credit for all the courses taken in the first BS program.  At the same time, some departments may have very strict requirements for what degrees they allow into their graduate program (these requirements may not be written anywhere and can vary wildly within an institution).

I would encourage you to reach out to your target department and talk with the Graduate Coordinator, Department Chair, or potential MS advisor.  I would explain your situation and come with a willingness to take background classes.
I would encourage you to take as much of the second half of the BS AE program as you can (the first half is likely redundant with the degree you have already).  You will learn a lot more by taking the classes in a normal manner - being forced to do projects / homeworks and getting to do the labs, etc.  This may well be the fun stuff you're most interested in.

After taking the background classes, you'll be able to proceed to a MS.  The courses will be more specialized, and you'll spend a lot of your time on your research project.  I would encourage you to find a university where a MS Thesis is the norm (vs. a non-thesis Special Project of some sort).  Unless you want to go all the way to Ph.D. -- in that case, don't worry about the MS Thesis and proceed...

Graduate education in the US is very expensive.  Even if you can afford it, you should pursue financial support.  That is the business of research universities and in general, if you aren't being paid to go there, you don't belong.

Thank you for your helpful comments; much appreciated.


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Would an 8% reduction in drag outweigh the weight addition from this custom CFD-tested winglet?
I'm writing an essay at school about why there is so much variation in winglet design. I am analysing all the primary types of winglets used today in the aviation industry. I'm constructing them in Solidworks (CAD software) and then running them through the in-built CFD. The winglets which gave me the lowest drag were those of the 737MAX. I then went on to design my own winglet and see how it compared - I took on a sort of compund spiroid/MAX winglet mix - according to the numbers fom Solidworks, the drag force with this winglet is 8% lower than that of the MAX winglet.
Now, OBVIOUSLY I am not better than the a team of hundreds of Boeing engineers with this design I stumbled upon, so I was wondering what the reasons would be for Boeing/an aircraft manufacturer not to use this type of winglet. I understand the consequences of weight on the wing root moment/structural reinforcements, etc... but I feel an 8% reduction in drag would outweigh the weight addition from this winglet?
Test parameters:
The wing is the same as that of a 737NG, with the winglet stuck on the end

TAS: 280kts
Air density: 1.2kg/m^3
Airflow: Incident at normal to fuselage direction
AoA: ~1.5 degrees

Images of the CFD and winglet itself are attached

How did you get an 8% lower drag by adding a vertical plate with profile drag?

@Koyovis I don't fully understand the physics of a spiroid winglet tbh! If by profile drag, you mean what Solidworks calculated, I have no idea! I'm a total novice to CFD so does the calculated drag force not incorporate drag from vortices behind the surface?

The CFD looks extremely crude, are the vectors shown just in some selected points or is the grid really so poor?

@HarryKarmel I find the result a bit sus to be honest, does it pass a sanity test? At first glance, your winglet looks like the existing winglet with a vertical plate added at the end. This vertical plate has drag. I am also not aware of any aerodynamic principle that more than offsets the added profile drag.

@Koyovis yh it has profile drag but I was working of the spiroid concept. I have no clue how these winglets give such great reductions in drag but I just added the plate to close off the circle and it seemed to work. So I think for the force it’s passed the sanity test in that it gives reductions in drag, but whether the amount to drag reduction is correct, idk.

280 knots indicated sound about right for cruise, but why did you run the simulation at sea level density rather than conditions representative of the actual cruise somewhere around FL360?

@JanHudec fair point! I started testing a while back on other winglets and I guess I just didn’t think it would affect the results that much so I left it as the default value. Seems kind of silly now! Though what effect would different air densities have on the vortices?

@HarryKarmel, both induced and form drag mainly depend on dynamic pressure, and you had appropriate dynamic pressure, so it shouldn't change that much. Compressibility effects come to mind, especially since the cold temperature (in ISA the temperature at tropopause is -56.5°C) means speed of sound is quite low, but I don't know whether the software you use can even calculate them.

I wanted to respond, but with so many detailed answers, not much reason. Just a note - I've used a lot of various FEA software - and Solidworks' built-in CFD is really only meant for quick validation. A real wing operates covered in bugs and ice particles, and in a mixture of laminar and turbulent flow. This has a lot of effect on its performance, but gets completely ignored by the low-fidelity model used.

@Therac yes there are some long responses! I’ve been completely blown away! Thanks anyway - I’ll still make a note of this in my essay

@Koyovis: The span increase of the added surface should explain the lower drag already. Now we don't know where on the polar those 8% are saved. Is it only induced drag? Cruise condition should imply that induced drag is 50% of total drag, and now we look at a 16% reduction in induced drag. This could be achieved with 7.7% more span already - and less added surface area.

The first possibility is that that the CFD in your cad isn't as sophisticated as the software used by boeing's engineers. Which means that your design may have flaws that don't show up in your software but do in Boeing's (or not even in there but do in a windtunnel).
Second is that I only saw one flight configuration being tested. Airplanes do more than just cruise at high altitude and the winglets must be good in all conditions. They must especially not affect the stall and spin behavior in a negative way.
Your winglets look kind of flimsy and I'd be worried about them getting ripped off or deforming in turbulent conditions. Those deformities would affect how they perform, possibly for the worse.

Those things look like they will come off in even a mild crosswind landing, tbh...

Thank you for your response. The dimensions aren't perfect, hence the thickness of the winglet! I don't have the time (nor the patience!) to test all flight conditions so my thinking was that on long sectors where fuel efficiency is most important, the plane will burn most of its fuel in the cruise phase, so are the other phases so important for fuel burn? An interesting point about the stall/spin effects. I read that blended winglets give the plane more control further into a stall, but make it harder to recover from one. No idea how my winglets will react but I'll add that to my essay.

Winglets are most useful in the climb to cruise altitude, and are therefore mostly used on mid-range aircraft like the B737/A320. The long range A350/B787's don't have them, simply because in cruise they are not as beneficial as a funky tapered tip shape.

@Koyovis The A350 does indeed have winglets, albeit blended ones. The 787 doesn't need them, as it could go straight span extension to get all of the wing it needed without running into the ADG V wingspan limits. The A350 couldn't just do a straight span extension, it that would have made it ADG VI, which definitely wouldn't be acceptable to airlines for a plane of that size and role. The 777X kept straight wings, but just has the outboard portion fold up after landing in order to stay inside ADG V. The 737 and A320 are both right on the limit for ADG III (the MAX is within 2 inches. - lol)

First off, awesome question and great investigation! This kind of let's-see-what-happens inquiry will take you far should you decide to pursue aerodynamics at an advanced level (and, of course, in other pursuits). Not so long ago, I had to write a similar report: lacking the resources and knowledge of the aerospace giants, I, too, wondered why I could seemingly invent designs that on the face of things appeared vastly superior to theirs. I thought that I had winglets down cold.
Then I went to work for Boeing and started talking to the aerodynamicists. I started graduate study of aeronautics. Turns out, unsurprisingly, that there's a lot you can't get out of undergraduate textbooks and publicly available data. While I obviously can't be exhaustive here—and probably won't even answer your question to the letter—I can give you a few things to think about. To be clear, I wouldn't go much further than you have with your modeling and simulation, but if you'd like some discussion points for your paper here are some in no particular order. I've made some assumptions about your level of knowledge, so please forgive me if it's patronizing and ask me if you need clarification.
The fidelity of your baseline model

The winglets...were those of the 737MAX....The wing is the same as that of a 737NG.

On what data did you base your model? The wing of a 737 is not a simple matter of an airfoil, some taper, and some twist. I notice that you did not include nacelles/pylons or flap-track fairings. The design of a production winglet is heavily tied to the integration of the overall wing design, including all of the extra components hanging off of it.
The reason that the 737 MAX winglets are effective
The 737 MAX uses what's branded as the Advanced Technology (AT) winglet. We know that a well designed wing extension is more aerodynamically efficient than a winglet. But the 737's wingspan must stay within certain limits in order to operate with the same ground infrastructure as previous models, so a winglet is a good solution. But what if we could have a little of both? Well, the AT winglet does exactly that:

The lower winglet is configured such that upward deflection of the wing under an approximate 1-g flight loading causes the lower winglet to move upwardly and outwardly from the static position to an in-flight position resulting in an effective span increase of the wing.

So to really understand the efficiency of the AT winglet, you'd need to model this deflected geometry.

The other contributing item to the effectiveness of the AT winglet is its natural laminar flow:

On previous winglets, the drag due to friction from the airflow over the winglet is one of the main detractors from efficient airflow....this is solved by Boeing using detailed design, surface materials and coatings that enable laminar – or smoother – airflow over the winglet.

The flow regime you're modeling
The AT winglets are most effective as their efficiency is aggregated over long, high-speed, high-altitude cruise legs. All you've given is a true airspeed, but for this kind of analysis of transport aircraft the Mach number is much more important. You haven't provided an air temperature, but from the density you've given it looks like this simulation is at sea level, which means your Mach number is not high enough. But this in fact might partially explain your results. Observe the drag curve:

In general, a spiroid winglet like yours reduces the induced drag at the expense of some parasitic drag. As you can see, we can afford some extra parasitic drag at lower speeds because the induced drag dominates.
If I were to make one suggestion, it would be to run your simulation at a realistic Mach number (around 0.8) and see what happens. But beware...
The limitations of your CFD software
We are getting to the point where CFD, when implemented well, is quite good for modeling aircraft performance in cruise flight. Much of the wind-tunnel testing for large aircraft these days is focused on high-lift and maneuvering conditions, where CFD falls much shorter. Of course, we always want to validate our CFD in the wind tunnel for all flight conditions, but for well-understood configurations in cruise the results often match well in terms of calculating overall performance. But the "when implemented well" caveat is key. I personally don't have experience with SOLIDWORKS Flow Simulation, but it looks like it's designed to be a general-purpose CFD software, so I would not trust its results too much for large, complex, high-speed simulations such as the ones required for this analysis.
In particular, there is the issue of turbulence. Not in the sense of unstable air that jostles an airplane around, but in the sense of chaotic flow over the aircraft's surface. So chaotic, in fact, that no computer in the world can accurately model the motion with a sufficiently short computation time. Instead, we use turbulence models that try to approximate what's happening in a way that can be solved quickly enough. SOLIDWORKS uses the k-epsilon model, which is popular for general-purpose software but might not be the best choice here. In particular, notes Wilcox,

Even the [k-epsilon] model's demonstrable inadequacy for flows with adverse pressure gradient has done little to discourage its widespread use.

As flows over airfoils are quite influenced by adverse pressure gradients, I would exercise caution. I can tell you that Boeing makes good use of the Spalart–Allmaras turbulence model in conjunction with detached eddy simulation (Spalart is an employee). But choosing the correct CFD implementation for a particular problem is a nuanced process requiring a great deal of judgement and care.

Wow! Thanks so much. This is really interesting - a lot of stuff I hadn’t previously considered like the design of the winglets to be effective with wingflex etc. You’ve given me so many talking points for my essay so I’ll certainly make a note of these!

The Spalart-Almaras algebraic model really originates in well-behaved boundary layers. But when used in DES it is good for its simplicity and because DES does the separation, not because the SA model is a good model for separated flows on its own.

First of all great analysis! I am not expert in aerodynamics but from the little I know, aircraft are a compromise. When you design a plane you have to make shapes that are possible to manufacture, don't cost too much and are strong (and compliant to the regulations). Last but not the least, you have to test the drag over multiple phases of the flight and in multiple configurations (flaps/slats): not only in cruise and analyse how this new design affects lift. Plus I don't know how accurate Solidworks CFD is: consider that the wind tunnel is still used because CFD is not perfectly accurate.
One thing I have forgotten: structures have a weight. If to make a complex shape you have to use stronger material, then the aircraft will have a higher weight that cancels out your gains in drag. What is the difference in drag between no winglet and B737 MAX winglet?

Thanks - I added some stuff in another comment about the phases of flight etc. Solidworks can't calculate induced drag directly so there won't be aerodynamic change in the CFD from weight gain. Funny you should mention the accuracy of the CFD too! While the shapes of the airflow might not be too far off, I feel the forces it calculates are wrong! The difference between no winglet and the original 737MAX winglet is slightly less drag for no winglets! Though the MAX winglets have 50% more lift which is strange (I think due to the upwash of vortices -wings with large vortices had negative lift!)

Where have you read about the 50% more lift? Indeed, I have just read (http://www.b737.org.uk/winglets.htm) that B737MAX winglets reduce the induced drag, so if it's not calculated this can be one of the sources of your inaccuracies

No that's just my seemingly sketchy CFD results although I'm not fully trusting the lift values they produce, because they have been very sporadic over the winglets that I've tested!

You may look at some NASA papers to get more accurate numbers and cite them in your report

thanks, yes I’ve cited some of the Whitcomb papers etc. I used this though because it gave me the freedom to test any design, not just those confined to NASA research

While all other points raised are valid, it is also important to go back to basics and first understand what winglets are actually supposed to do, and what drives the performance of an aircraft:
Winglets do not reduce 'profile drag', or the drag force that exists when there is no lift being produced. They more than likely will increase profile drag slightly. If your reduction in drag is coming from a reduction in profile drag, this is immediately suspect, bordering on perpetual motion machine crazy. Winglets generally reduce 'induced drag', which is the portion of your total drag force that is produced when the wing starts creating lift. How much induced drag is produced depends on the efficiency of the wing and its span, or an 'effective span' calculated by multiplying the physical span by the wing's efficiency. Winglets are supposed to increase the wing's 'effective span' without exceeding physical span limitations. This means that the reduction in drag will be higher the more lift you need to produce, or the higher your angle of attack. Because large airliners are typically designed around a few main cruise conditions at relatively low angles of attack, they typically claim improvements in the order of 2-4% on fuel efficiency from the addition of winglets.
It is important to compare drag values at equal lift values rather than at equal angles of attack. At a certain angle of attack, winglets can be effective by either reducing the drag, or increasing the lift (usually by sneakily increasing the physical span a little bit), or both. The important performance parameter that you should be plotting is the drag polar, where lift is plotted against drag. An aircraft will cruise at whatever angle of attack is required to maintain 1g, which will depend on its weight and lift characteristics. Angle of attack does have large implications on the design, but it does not drive performance. When comparing different configurations, overlaying drag polars will tell you everything you need to know, including how the profile drag values differ. You should run your simulations at differing angles of attack and plot the resulting lift coefficients against the resulting drag coefficients. A quick check on the case you have already run would be to make sure your drastic reduction in drag isn't accompanied by a drastic reduction in lift. Because your main lifting surface has not changed, this should not be the case.
Everything everyone else is saying about the occasional inadequacies of cfd, especially 'quick and easy' solutions built in to CAD packages is true, but I would still not expect such a drastic difference between two relatively similar configurations run on the same software. I would make sure absolutely everything else in the simulations of the different configurations is the same, check the changes in profile drag and lift, and do some flow visualisation to try and understand what is happening in the simulation to cause the drop. Then I would check if the same thing happens at a higher Mach number. Ideally you would want to check other software packages, but i understand this may not be possible and may be outside the scope of your essay.

Thanks for your response - I'll bear this in mind. The actual aim of the essay is to examine and compare multiple designs, so I 've got lots of data for different winglets for which I'll just do a side-by-side comparison. Ultimately, this isn't a scientific research paper so I'm not entirely bothered if the numbers aren't completely correct! Though I may try the different AoA/Lift coefficients for this winglet and see what happens! Thanks

I would say the biggest thing is that the computer model has to be validated with wind tunnel results. As anyone truly familiar with the climate debate knows, computer models trying to simulate extremely complex phenomena often at some point start to diverge from reality and you still need to test in the real world to validate or falsify the model to really find out.
I would bet then that your version performs well because the modelling is inadequate at reproducing some subtle effect, and that if you tested it in a wind tunnel you'd be disappointed.
The structural analysis is a bit similar. Computerized finite element analysis done for the structure for strength and endurance has its own limitations and still has to be validated with a run in an endurance test rig, and very often the computer's predictions are wrong.  This is why it's important to get the structural endurance rig testing completed as early as possible in the production program to minimize the patches that have to be done in service when testing uncovers inadequately modeled structures.

Yeah I don't have the resources to do a full wind tunnel test etc..! But this is an interesting note that I can add to my essay. I'll also try some FEA on the component too - I think Solidworks is supposed to be better at FEA than CFD - thanks!

Firstly congrats on your ‘out of the box’ thinking and an interesting concept!
The previous answers have hit a lot of the major points so apologies if I repeat any of them, but here are my initial comments:

All wing tip devices (or extensions) reduce lift-induced drag. The operating condition heavily affects the amount of induced drag present. There is a quadratic relationship between induced drag and the lift coefficient (CDi=CL^2/PI*AR). Therefore, it is important to characterise the performance of your wingtip across a representative envelope. There will be a cross over point at low CL’s where your wingtip produces a penalty due to viscous losses and form drag.
Operating CFD analysis in delta mode requires that you have a good representation of the baseline aircraft. I suggest you test out your modelling methodology on a representative Common Research Model (CRM) that has wind tunnel data available for validation. The Drag Prediction Workshops are useful for this (https://aiaa-dpw.larc.nasa.gov/Workshop4/workshop4.html) they provide geometry, grids, wind tunnel data and you can see the spread of other parties CFD predictions. If you can’t get sensible figures for the baseline you know that your approach needs work.
Your comparison should be carried out at lift matched conditions not alpha matched. This can be achieved by either carrying out an alpha sweep for both models and interpolating or depending on your solver you may be able to allow alpha to float and set a fixed CL (this is what we do).
Wing root bending moment penalty is an important consideration also as the additional loads from a wing tip extension usually require additional spar strengthening which in turn adds weight and eats into your performance gain.
Another consideration is the additional pitch down moment generated from your wing tip device. As the tip is generating lift and is located aft of the aircraft CoG there will be additional pitch down moment. This will have to be trimmed out by the horizontal tail fin which will incur an additional drag penalty which again will reduce your performance gain.
The aerodynamic efficiency is usually quoted in terms of Lift/Drag ratio. I noticed from your model images that you are simulating just the wing and fuselage, which is fine but for the L/D calculation, you should take into account the missing components: horizontal tailplane, vertical tailplane, engine nacelles, engine pylons. As the lift over drag calc calls for a division over the drag this operation is non-linear. Flat plate drag theory can be used for estimating the drag contribution of missing components. This will make the performance analysis more representative of a full aircraft configuration.
Handling characteristics are also an important area of consideration, for example: buffeting, stall behaviour, yaw response etc. That’s a very large area to get into so I won't!

Anyway, you can see from the brief list above that there are lots of considerations that need to be taken into account for a viable wingtip technology.
I hope this helps, keep up the good work!

Thanks so much! I’ll be taking this into account - I’m planning on touching upon the stall characteristics etc in the essay but detail on the designs. That’s an interesting point about the CRM comparisons. I’ll have to have a look at this!

authors: Vladimir F Героям слава


text:
If an elliptically-shaped aerofoil gives the optimum lift distribution, why aren't propeller blades designed around this ideal?
So given an elliptical lift distribution is the most efficient, why are propellers not designed around this? Props aren't affected by the adverse stall characteristics that concern a "main", elliptically-shaped wing, given they (theoretically) should never stall.
The picture of a C208 propeller below illustrates what I mean; the propeller is raked at the tips. Is this related to encouraging a spanwise flow due to the tips approaching Mach 1? I would have thought stubby propeller blades like those wouldn't reach speeds where wave drag becomes an issue?

I suppose this P-51 propeller is closer to that ideal shape but it is still relatively square at the edges. I understand that you couldn't strap a perfectly elliptical wing to a propeller given the high aspect ratio needed of the blades, but nevertheless, I don't understand why propellers are not at least approximated to the ideal shape...

Consider changing "aerofoil" in the title to "wing" or "wing planform".  As it is, it sounds like you are talking about the choice of airfoil, i.e. the cross-section.

Be aware that even for wings it is elliptic lift distribution, not elliptic shape of the wing.

If an elliptically-shaped aerofoil gives the optimum lift distribution …

I interpret this as an elliptically shaped wing, because only then the question makes sense.
Propellers are similar, but not identical to wings.

Dynamic pressure varies along blade span. The outer sections need less chord for the same lift compared to inner sections.
Propeller blades operate at the start of a helical wake, so induced speeds are different than those on straight wings.

What is required is not an elliptic lift distribution, but minimum induced losses. Albert Betz showed in 1919 that the optimum induced speed is constant over the propeller disk, just like downwash is ideally constant over span for minimum induced losses. Based on this, Larrabee found an efficient computation method and practical propellers have to find a compromise between theory and practical constraints. The most important are

Compressibility effects near the tips (which force thinner airfoil and lower lift coefficients than what the ideal prescribes). An extreme case is the Junkers VS-9 propeller used on the Ta-152.
Structural limits at the root (which force the use of much thicker airfoils).

Betz' theory is meant for lightly loaded props. Once disk loading goes up and the propeller diameter cannot grow proportionally, propeller chord and blade count grows and the main concern shifts to converting all available power to thrust. Concerning fancy blade shapes: They have more to do with marketing or unscientific voodoo beliefs by some hobbyists. Generally, a straight shape works best, unless you desperately try to reduce compressibility losses by sweeping back the tips.

The different parts of a propellor blade are encountering air moving at different speeds.  The tip moves through the air much faster than the parts near the hub, so it's not surprising that the same airfoil is not optimum at all points.

The propellor blades also need to be designed to handle the forces caused by rotating at a few thousand RPM.

YES, these two things.  Exactly what my answer was going to be.  The question considered only uniform airflow and neglected the structural aspects.

This does not answer your question however I feel the need to correct an assumption you've made.
The body of your question is correct:

So given an elliptical lift distribution is the most efficient

However, the title of your question is wrong:

an elliptically-shaped aerofoil gives the optimum lift

It is not the elliptical shape that gives best lift at lowest drag. It is the elliptical lift distribution. Lift distribution means varying the amount of lift along a surface.
There are three ways you can vary lift:

Change angle of attack

Change airfoil

Change the chord size

You seem to assume that a lift distribution that results in an elliptical graph when plotted on paper/computer screen implies that the flying surface needs to vary its chord so that it forms an elliptical shape. Doing this you have forgotten the first two ways that lift can be altered.
Yes, in the old days before computers and parametric CAD software it was really difficult to manually calculate how to vary the wing's angle of attack (twist) or airfoil to get an elliptical lift distribution. Therefore aircraft designers did the simplest thing: made the wing shape elliptical.
If you look at modern high performance aircraft such as airliners you will see that physically their wings don't look elliptical. However, since fuel consumption is an important factor for airlines, manufacturers still design the wings to have an elliptical lift distribution. Only these days it's done by varying the wing's twist (washout) and can be calculated automatically by CAD.
So even a straight rectangular wing can have an elliptical lift distribution.

authors: Vladimir F Героям слава


text:
Do Air Passenger Number statistics on a route refer to both directions of flight?
Looking at the air passenger data for specific routes, I was wondering if this data referred to just one direction of the route, or both directions?
For example (see link below), Philadelphia's most served route is to Orlando, flying 548,320 passengers last year. Is that number referring only to PHL-MCO, or is that cumulative of PHL-MCO and MCO-PHL?
Thanks!
https://en.wikipedia.org/wiki/Philadelphia_International_Airport#Top_destinations
https://www.transtats.bts.gov/airports.asp?pn=1&Airport=PHL&carrier=FACTS

By looking through the BTS website you linked (and is cited in the Wikipedia listing), you can find the answer.
The first 3 sections on the infographic; Summary Data, Carrier Shares, and Total Passengers include both departure and arrival passengers totaling about 14 million passengers.
The fourth section Top 10 Destination Airports (US Only) is less clear and you can't cross-check to the other totals because it is limited to only the top 10 airports. But it is a clue that the title says Destination Airports which implies departures only.
But there is a potential check. At the top, you can select the airport. By selecting Orlando, FL: Orlando International from the pulldown, you can see the data from there. That shows in the Top 10 Destination Airports that PHL is number 3 and the total number of passengers is 539,900. Since this is different than the 548,320 PHL-MCO number, it would appear that section references departures only.


text:
Possible military aircraft ID, N. Carolina
I was recently in Oak Island, NC and saw the following plane flying above. It was low-flying and much louder and larger than the other small prop planes flying out of nearby Cape Fear Regional Jetport. Based on the size of the propellers I suspected it was some kind of military aircraft but wasn't able to figure out what it was. I know this is a small, dark picture, but does this plane look familiar?

This looks like a V-22 Osprey.  Here is a photo.  (it happens to be refueling, so ignore the nose hose.)

This is not a 3D render ?

@Antzi: Even if it is a render rather than an actual photo, what difference would it make? It shows an Osprey.

From the color, it looks like a CV-22. Definitely a V-22 though, and the differences are subtle.

Also, definitely a render. Some of the colors are wrong and the intakes are too clean.


text:
How can I find out if the FAA has ever sanctioned a pilot?
Is there any easy way to check to see if a CFI or commercial pilot has had past problems with the FAA?  The question came to me after an old thread in one of the aviation forums pointed me to this crash report:  https://www.wfsb.com/news/waterbury-flight-instructor-among-killed-in-ny-plane-crash/article_06e41c00-d066-11e8-a98c-7beca823cc54.html
The report says the pilot had had several enforcement actions against him, including having his CFI certificate revoked for falsifying logbook entries.  My guess is his two passengers would have liked to know this before going on their fatal flight.  The FAA certificate database only shows his ratings, not the enforcement actions.  Where else could they have looked?

The FAA maintains a database of appeal actions, but I think enforcement actions can only be located through a FOIA request. I could not find anything online that shows enforcement actions against a pilot. That said, customers can always ask before engaging the services of a pilot.

I asked the FAA if there is such a database. I'll let you know what they answer.

I asked FAA Airmen Services about this. There is no easy way to do what you want, as there is no online database that can be queried for pilot enforcement actions. They said you can send them a query by mail or email using this format:
                       In accordance with the Privacy Act

    Date:

    TO:
    FAA
    Aviation Data Systems Branch, AFS-620, ATTN:  Privacy Act
    PO Box 25082
    Oklahoma City, OK 73125
    Email:  9-amc-afs620-pa@faa.gov

    FROM:
    COMPANY NAME
    NAME OF COMPANY REPRESENTATIVE
    STREET/MAILING ADDRESS
    CITY STATE ZIP CODE
    Email address:  <Please enter your email address here>

    Under the authority of the PRIVACY ACT, I am requesting verification of
    current airman medical certificates, current airman certificates indicating level,
    category, class, and associated type ratings, including any limitations to those
    certificates and ratings and any and all information concerning FAA
    accidents/incidents, and/or FAA enforcement (AIE) history for the following
    airmen:

              Name:                              Date of Birth:            Certificate Number:
        1.
        2.
        3.
        4.
        5.
        6.
        7.
        8.
        9.
        10.

I understand that the response to my request should be mailed to me within 20
federal working days.  If you have any questions, please contact me at:  XXX-XXX-XXXX.

Sincerely,

I am surprised there isn't a faster mechanism for this. Enforcement actions should be public record, same as criminal conviction records. But I guess this process is geared towards background checks of pilots by potential employers, not the general public. Odds are it would take an act of Congress to change this.

I suspected the journalists in the article went this route.  Thanks for the extra legwork.


text:
Planes returned to base after dumping fuel due toilet issues
A colleague of mine experienced this and I was astonished.
Flying KLM from PVG to AMS a couple of hours into take off toilets became full. The pilot dumped fuel and returned to PVG. All passengers were re-booked next day. Funnily, she had the same issue fly from JFK to LHR last year. (Probability that she was on both planes - amusing)
Thinking of aviation fuel, environment, cost of re-flying passengers. I wonder why the aircraft makers not allow dumping 'contents' of 'toilet tank' (on the open sea) instead of dumping gallons of fuel.
I just wonder how often does such issues happen? Would toilet tank emptying not be checked before take off?

I'd guess the tanks weren't full, since they are emtied before flight. But there might have been a problem with the... content delivery system to the tanks.

An aircraft with full lavatory waste tanks can not continue on it's flight as it is a requirement to have a specified number of serviceable toilets on every flight.  It is usually the responsibility of the Purser or In Charge Flight Attendant to check the waste tank volume when they prepare for each flight. On most aircraft there is a control panel with a display that shows the actual waste volume.
Here are some possible reasons why the waste tanks became full:

They were not checked before the fight
They were not emptied on the ground
They malfunctioned and ran continuously
The display on the waste control panel malfunctioned

In any case these tanks can not be emptied while airborne, and the only course of action is to land and have the tanks emptied or serviced.
On a long haul flight like PVG to AMS the crew would not have been legal to continue their duty day after returning to PVG and the flight would have to be cancelled or delayed until a new rested crew could be found to operate the flight.

FYI: 1. gets you nice list formatting while 1) does not. Minor detail.

Airliner toilets cannot be emptied in flight. The valve is on the outside, and can only be accessed by ground crew. Besides, two hours of flying would not have filled the toilets. It is much more likely that the ground crew at PVG forgot to empty them before the next leg to AMS, or their toilet service cart was not working at the time.

I wonder why the aircraft makers not allow dumping 'contents' of 'toilet tank' (on the open sea) instead of dumping gallons of fuel.

If you are implying that the waste should be dumped to allow the flight to continue, then the additional complexity, weight and cost that would go into a mid-air waste-pumping device makes it completely impractical.
The waste tanks are intended to be big enough for any flight, and should start empty if the ground-crew handles it properly.  A waste-pumping mechanism would need certification and regular maintenance, and take from the weight and fuel capacity of the plane. It would very rarely get used, and is better handled just by reliable ground service.

I think the OP meant why can't the waste tanks be dumped so that the toilets can continue to be used instead of aborting the flight.

@ward The above comment answers that question.


text:
Is it possible to get a list of Cl/Cd values for various NACA Airfoils?
There are a lot of links which give the shape of the airfoil for any given set of camber thickness, position and max. thickness values, but I would like to know if it is possible to get the Lift vs. Angle of attack graph, i.e., in turn lift curve slope, and also Cl/Cd values for every NACA airfoil specified.
Actually, I needed the values to begin the designing of a basic propeller blade for my project.
I have tried searching a lot. It would be really helpful if anyone could help me proceed.

I suggest you look into Xfoil: http://web.mit.edu/drela/Public/web/xfoil/, it's freely available software that is great for low speed modeling.

Perhaps the most important resource for airfoil data is the NACA (the predecessor of NASA) Report No.824 Summary of Airfoil Data by Abbott, Von Doenhoff and Stivers.
This report contains the description, generation and experimental data like variation of lift, drag and moment coefficients with angles of attack for a number of NACA airfoils.
You can also see NACA report No. 460 The Characteristics of 78 Related Airfoil Sections from the Tests in the Variable Density Wind Tunnel by Jacobs, Ward and Pinkerton.
Also, Aerodynamic characteristics of a large number of airfoils tested in the variable-density wind tunnel by     Pinkerton and Greenberg.
These are similar to the first report, but older.

Theory of Wing Sections by Abbot and Doenhoff is a good one. I use that book to select my aerofoil and aerodynamic characteristics.

@J.Hougaard: Which link? there is no link... (I known this comes from your choice in the review queue)


text:
What are the advantages and disadvantages of having a single castor wheel as compared to 2 wheels in an airship?
I am working on a project to create a mid-size airship having a payload of 25kg. I am in a bit of problem regarding choosing the appropriate landing gear design for it.
Which design would be better, considering the tradeoff with weight, which is a very important parameter. Is using 2 wheels to stabilize my system further worth its weight penalty, or a single wheel would be better like Zeppelin?
I would appreciate any  help.

An airship that small doesn't need landing gear at all. Put a skid on the belly of the gondola and call it good.

True. But the project is just a model which we plan to scale up.

Two wheels side-by-side would only be needed if the craft is in danger of tipping over sideways. Airships should stay upright because the lifting gas will still provide most of the lift, and the wheel is only for carrying the remaining load. It can swivel for ground maneuvering when the airship needs to swing into the wind while mooring at the mast, but will neither absorb big landing loads nor carry the craft at high speed, so it can be kept small.
On the lower tailfin you should only provide a bumper for unintended ground contact; adding a second wheel would only make sense when the ship touches the ground with both wheels when level. This would be a bad design, however, because maneuvering will be much easier when the lower tail fin provides enough ground clearance to allow pitch angles of approx. 3° - 5°. Note that the lower fin on most Zeppelins was smaller than the upper fin; only the last, less slender ones could afford equally sized fins. In that case, however, you need sufficient propulsion power to keep the cruise speed well above the reversal speed.

"Airships should stay upright because the lifting gas will still provide most of the lift": If you never plan on deflating the airship, then sure...

Some full-sized airships have three wheels in a tricycle arrangement, such as this wikipedia photo link.
Others have just two, such as the Zeppelin NT - one for weight bearing, and a smaller tail wheel which appears to be there to keep the tail from fouling on the ground.
I'd suggest, for the size you're planning, you just go for the minimum possible.  Only worry about wheels if you will need to taxi it or steer it into wind.

The tricycle arrangement is only for ground handling with deflated envelope. For operations, a single wheel should be sufficient.

Photo of a Zeppelin NT on the ground: http://www.efalk.org/Eureka/bigs/dsc_001.jpg.html